Many electric devices contain two loads that must separately controllable. An example of such a device is a humidifier that also includes a night-light. The humidifier is controlled by an automatic time switch, so that the applicable pumps and fans are always switched during a specific time period in which, for example, the operating noise is not disturbing. Other controls apply to the night-light. It is controlled as a factor of readings taken by a light sensor, for example, so that the night-light is only switched on when it is dark.
To solve this task, a circuit array for controlling two independent loads operable with rectified AC voltage is used in electric devices. Although electric loads, such as lamps, are also operable with AC voltage, it is advantageous, in terms of switching the loads, if the voltage in question is DC voltage. Said DC voltage can be switched with a MOSFET or a bipolar transistor, whereas the control of AC voltage-operated loads is significantly more complex, because both the positive and the negative half-wave must be taken into account. The controlled loads are generally operated with the standard power supply, because, on the one hand, a lower operating voltage would require a more expensive transformer and, on the other, the current that occurs would be considerably higher, which would require thicker cable diameters. Thus the entire circuit array, and therefore the device, would become more expensive. In the case of a 230-volt power supply, the peak voltage for rectified voltage is 340 volts. Therefore, the types of transistors used to switch loads must be capable of switching this high voltage. Thus, in the case of circuits based on the prior art, two semiconductor switches suitable for high voltage must be used, which are very expensive in comparison to the operating circuit and thus constitute a high proportion of total manufacturing costs. For this reason, the high price for the high voltage semiconductor switches used also constitutes a high proportion of the costs of the entire device, because electric devices of the type described are simply designed devices that are sold at very low prices.
A circuit essentially known in the art is shown in FIG. 1, and will be described in greater detail in connection with the description of the figures.
Because the semiconductor switches suitable for high voltage constitute a high proportion of total manufacturing costs, it is desirable not to be forced to use two semiconductor switches for the control of two loads.